Shadow Darners (Family Aeschnidae)

December 29, 2015

Salutations, BugFans,

What better way to end the old year than with a spectacular dragonfly (charismatic megafauna), and many thanks to BugFan Freda, guest photographer.

The BugLady is yearning for a Shadow darner. The books say they’re common, and other people are up to their ankles in them, but not the BugLady, although she scared up some big, dark darners in early fall that didn’t stick around to have their pictures taken. She has read that Shadow darners may collect in small groups on tree trunks, but really, one would be enough.

Darners are large dragonflies in the family Aeschnidae. They’re big-eyed and powerful dragonflies that are sexually dimorphic (males and females look different), and the females of many species are called polymorphic (many forms, because they come in several different color phases). Mosaic darners (genus Aeshna) get their names from the blue “mosaic” patches on the abdomens of the males. Caveat—if you’re using a camera instead of a hand lens to identify some of the mosaic darners, the ID is a “probably.”

Shadow darners

Shadow darners (Aeshna umbrosa) live throughout most of North America (except the very southern edges of the U.S. and a few Rocky Mountain states), and their range stretches well north into the boreal forests of Canada. They’re found in a variety of wetlands, from the still waters of bogs, pools, and ditches, to slow streams. There is an eastern subspecies (Aeshna umbrosa umbrosa) and a western one (A. umbrosa occidentalis), with slight differences in coloration (the former has small, green abdominal spots and the latter has blue ones). See an awesome comparison of the subspecies, at https://www.flickr.com/photos/gyr/sets/72157614135970414?view=sm.

Overall, this is a large, brown dragonfly (some sources refer to it as dull) that’s around 3″ long with a wingspread close to 4″. The all-important lateral thoracic stripes are straight and generally pale, and an Ohio DNR wildlife website says that “the green mark on fore part of thorax often resembles the Nike ‘swoosh.’” The face is pale and lacks a conspicuous black stripe across it (males may show a hairline stripe), and the male’s cerci, described as “wedge-shaped” by some references and “paddle-shaped” by others, are spine-tipped. Females’ wings may be brown-tinted. Some sources say that the thoracic stripes are outlined in black—not an “in flight” field mark (but see link, above).

As their name suggests, Shadow darners spend most of their lives in shady woods and edges, and they may fly until it’s too dark to see them (though they’re more likely to be active during the daylight when the cooler weather of fall sets in). Shadow darners are associated with the tail end of the dragonfly season; almost three-quarters of Wisconsin sightings are in August and September, but there are May records and they are seen well into October.

Shadow darner female

Shadow darner male

What do bumblebees and Shadow darners have in common? They push the limits of cold-bloodedness, remaining active in very cool temperatures, when other dragonflies are grounded. Odonates use a variety of strategies to regulate their body temperature—passively, by basking to collect heat or assuming the obelisk position to avoid it; and actively, by contracting the wing muscles/quivering their wings while perched (wing whirring), to warm up the flight muscles (and therefore the thorax), and also by slowing circulation to the abdomen in order to keep heat in the thorax instead of sending it to the abdomen, where the larger surface area allows cooling). Their colors may darken in cold weather, and dark colors absorb more radiation from the sun. An overheated Shadow darner may immerse its abdomen in water (water dipping).

Shadow darners are agile and active flyers, scooping small, soft-bodied insects (and the occasional fellow-Odonate) out of the air into legs arranged like a basket, discarding the wings, and feeding in flight. One source said that they consume as much as 20% of their bodily weight daily. They sometimes form feeding swarms or join other darners in mixed swarms. They are, in turn, fed upon by raptors, especially the smaller falcons, and by purple martins. Ovipositing females may fall prey to frogs (one well-annotated source added predation on females by salamanders/newts, which raised the BugLady’s eyebrows. She consulted her herp guy, and he couldn’t picture a salamander taking on a large darner, either—a salamander lucky enough to grab a dragonfly doesn’t have the equipment to process it into smaller pieces (and many salamanders are in the 3” to 4” size range, themselves) (thanks, BugFan Tom). Naiads, like their elders, are unapologetic carnivores, feeding underwater on any aquatic invertebrate (or small tadpole, larval salamander or fish) that they can wrap their labium around, and being fed on by a variety of parasites (therefore, warns one site, do not eat dragonflies!) and by fish and birds.

There’s not much of a courtship, and even less of a honeymoon. Females mate with the owner of the territory they enter (or they don’t, arching their body in the opposite direction to show lack of interest). He takes her to a promising spot, they separate and she oviposits alone, while he guards his investment from the air.

Paulson, in Dragonflies and Damselflies of the East, describes the Shadow darner’s strenuous reproductive activity.

Males fly beats up and down streams and along lake shores, with much hovering while facing the shore, even as long as 30 sec in one spot…May patrol and defend entire small pond, usually for a period of less than 1 hour, and typically move from one patrol area to another, often at different water bodies… Females oviposit on logs and twigs in water or on moist tree trunks or earth banks, sometimes well above water and even in rather dry situations. Less likely to use living plants than most other darners. Perhaps because of woody oviposition substrates, females much more likely than other mosaic darners to break off cerci as they mature.

In cooler climes, eggs hatch the following spring—nd in cooler climes, naiads may overwinter until the year after that, emerging in early summer.

Interesting Shadow darner factoid: Shadow darner naiads are sometimes introduced into rice fields as a biological control of mosquitos (a task at which their elders excel when both the dragonflies and the mosquitoes emerge from water into adulthood).